2 Answers
2

Through permissions there is no way to differentiate between being able to add files and add folders.

But you still have two options:

If your document libraries are set up with the right folders and nobody should add new folders then you can just go into Library Settings|Advanced Settings and set Make "New Folder" command available? to No. If you then later need to add new folders, then someone with Manage Lists permissions can open a window by setting it to Yes, Create the folder and resetting it to No.

You can write an Event Receiver which denies any attempt to add a new folder based on your own logic.

To enable or disable folders in lists and libraries, you go to the list or library settings page by switching to the List ribbon or Library ribbon and clicking the List Settings or Library Settings button. On the settings page, you click the Advanced Settings link to get to the Advanced Settings page of the list or library . On that page, you choose Yes or No for the Make New Folder Command Available option, which is located in the Folders section of the page